Is the rivet that shocks you the only one with damaged/no paint/bare metal? If so it might be because the paint is insulating. In any case take a voltmeter and probe that rivet (other probe should be in the ground of an outlet). And the snack machine as well, should be voltage on whatever machine has the issue.

If it has an SII board probably. If it’s single price (no control board and no led display) then you would need all new harnesses, which would put it solidly in the not worth it category. It is possible, just not worth the time and money.
Personally, I would only convert single price to multi price if it was a single price 501E, and I had a donor SIID machine that had severe door and cabinet damage to pull all the harnesses off of.

Get a Mars amusement validator AE series. This will be able to send simple pulses to the machine. MDB is a complex protocol and so will not interface to your machine, which takes simple pulses to work (typically 1 or 4 pulses per dollar).

You can turn it on in the options.
http://www.vendingworld.com/Manuals/Dixie/S2D-Program.pdf
Page 11. It’s called SSM or Scrolling Sales Message. The message itself is “ICE COLD DRINKS”. On units built for Coke it should say “ICE COLD COCA-COLA”.

Make sure they are Bev Max 4s and not 2s or 3s. If they are 4s and properly refurbished that is a golpher smoking deal. As in I would be making calls to line up a semi to haul them to my shop right now and working out how to pay the seller.
Even if they are just good used and not refurbished if they are Bev 4s that is still a decent deal.

If the coin falls through to the coin return then that means the mech is not getting any power, or is broken. As such, it will not set a credit on the machines credit relay when the vend price is reached, which is why the doors don’t open.

Unfortunately the Go 380 is an especially bad machine, even for an import. While most other imports have the decency to use an MDB coin mech and Validator, meaning you can at least replace them with something reliable, the GO380 uses a cobbled together Frankensteined in house proprietary POS. As far as I can tell, the acceptance portion is a crappy arcade mech and the payout area is cheap stamped sheet metal. I'm guessing that your arcade mech failed.